Spherical reflector antenna with waveguide line feed



1970 J. J. GUSTINCIC ETAL 3,

SPHERICAL REFLECTOR ANTENNA WITH WAVEGUIDE LINE FEED Filed March 22, 1968 2 Sheets-Sheet 1 PLANE WAVE FRUIT INVENTORS ALLAN W. LOVE B JACOB J.GUSTINCIC ATTORNEY Och 1970 J. J. GUSTINCIC ETAL 3, ,373

SPHERICAL REFLECTOR ANTENNA WITH WAVEGUIDE LINE FEED Filed March 22, 1968 2 Sheets-Sheet 2 FIG. as

omecnou 01- cemsn or cunvm'uns or REFLECTOR mm SLOTTED, TAPenm WAVEGUIDE RADIAL FINS omecrlou OF REFLECTOR mam ramsmon srmmnsn ROUND WAVEGUIDE TE H none INVENTORS m LOVE Au. w. .1111 JACOB J. GUSTINCIC United States Patent 3,534,373 SPHERICAL REFLECTOR ANTENNA WITH WAVEGUIDE LINE FEED Jacob J. Gustincic, Los Angeles, and Allan W. Love. Corona Del Mar, Calif., assignors to North American Rockwell Corporation Filed Mar. 22, 1968, Ser. No. 715,346 Int. Cl. Hfllq 13/10, 15/14 US. Cl. 343-761 16 Claims ABSTRACT OF THE DISCLOSURE Pivotable line source feed means for cooperation with a fixed spherical reflector as a focused source of a scannable pencil beam, useful in radio astronomy and deep space tracking applications. A leaky cylindrical guide, having radial (disc-shaped) fins interposed between axially spaced rings of radiating slots, is oriented along a focal axis of the reflecter. Preselected tapering of the Waveguide radius corrects for aberrations associated with the finite line source and spherical reflector. The slot areas are preselected from ring to ring to achieve a desired aperture amplitude distribution (say, tapered or gabled for sidelobe reuction); and the radii of successive fins are preselected to disturb the normally mutually quadrature phase relationship between the E, (azimuthal) and E (axial) components for improved aperture efliciency.

BACKGROUND OF THE INVENTION As is well understood in the antenna art, a paraboloidal shaped reflector, in cooperation with a point source microwave feed located at the focus of the paraboloid, will provide a collimated antenna beam pattern or planar wavefront having good directional properties. The beamwidth may be made narrower, ro the directional properties improved, by employing a larger aperture or larger reflector. Directional scanning or beam-steering of such an antenna is conventionally achieved by mechanical rotation of the reflector and microwave feed as a rotatable rigid assembly.

For radio astronomy and deep space tracking applications, extremely large antenna apertures are required to achieve the narrow beamwidths of interest, aperture diameters in excess of 300 feet not being unusual. However, beam steering by means of mechanical scanning of paraboloidal reflector assemblies of such size is prohibitively expensive. An alternate beam steering method contemplates the use of radio energy feed means pivotably mounted at the focal point of and relative to a fixed reflector. In order that the steerable beam have like beamwidth properties in each steered direction, a common curvature is required for all sectors of the reflector, which requirement is fulfilled by a spherical surface having a rotatable feed at the center of the curvature.

Such an arrangement has the Well-known disadvantage that a spherical reflector does not have a point focus, but rather a line focus, referred to as spherical aberration. Such effect may be compensated for by use of a distributive feed source along such line focus. Such compensatory distributive radiating feed means are described in an article by A. W. Love in the I.R.E. Transactions, vol. AP-lO, No. 5, September 1962 and in US. Pat. 2,997,711, issued Aug. 22, 1961, to A. W. Love for Spherical Reflector and Composite lllumiuator.

The elimination of such spherical aberration, while necessary, is not sufficient to provide either a satisfactory beam pattern by means of a spherical reflector or high aperture efliciency. In other Words, such spherical reflector antenna may yet demonstrate undesirable sidelobe patterns; also, the nominal aperture efficiencies associated F CC with prior art feeds limit the tracking ranges and system signal-to-noise ratios of systems employing such feeds.

Further for example, the longitudinal slotted, tapered rectangular feedguides disclosed in the US. Pat. 2,977,711 do not provide a uniform or circular field pattern in a given plane perpendicular to a given station along the feedline whereby a true pencil beam or narrow circular beamwidth is not obtained. Because of this non-circular beam pattern, aperture amplitude tapering efforts for reducing sidelobe performance are of limited and non uniform effectiveness. Also, the use of a composite feed, incorporating both a line source feed and a point source feed add to the complexity of the design, the polyrod radiators employed as a point source for paraxial illumination of the reflector also providing a source of increased aperture blockage. Moreover, the use of a mulitple-channel rectangular line fed containing discrete phase shifter Wedges and power splitter partitions further adds to the over-all design complexity and cost of such structure.

SUMMARY OF THE INVENTION By means of the concept of the subject invention, the disadvantages of such prior art spherical antenna feeds are avoided, while the advantages thereof are retained.

In a preferred embodiment of the subject invention, there is provided a mechanically-scannable reflector-type antenna comprising a reflector of spherical curvature and line source feed means pivotable about a center of curvature of the reflector as a focused source. The line source feed means comprises a preselectively leaky cylindrical waveguide for providing a uniform field as a function of azimuth angle about the longitudinal axis of the feed, the guide further being preselectively tapered to reduce aberrations associated with the cooperation thereof with the spherical curvature reflector. Polarization sensitive phase shift means axially spaced along the line source adjusts the normally mutually time-phase quadrature reationship between the azimuthal and axial near-field components of microwave energy radiated by said waveguide for increased aperture efliciency. Preselective adjustment of the leaky waveguide radiating apertures as a function of axial position provides adjustment of the antenna aperture amplitude distribution for reduction of sidelobes. Further, each of such compensatory structural features may be independently adjusted, whereby an antenna of optimum design may be conveniently achieved. Moreover, a point source feed for paraxial illumination is not employed, thereby reducing aperture blockage and effect ing further economies in design.

Accordingly, it is an object of the invention to provide an improved mechanically scannable spherical reflector antenna.

It is another object of the invention to provide a spherical reflector antenna having a focused feed which is mechanically scannable relative to the antenna reflector and of economic design.

It is still another object to provide a mechanically scannable spherical antenna having reduced sidelobes and increased aperture efliciency.

It is a further object toprovide a spherical reflector antenna for radio astronomy applications and for which compensations for spherical aberration, sidelobe performance and aperture efliciency may be independently adjusted.

It is still a further object of the invention to provide a spherical antenna with a mechanically scannable focused feed of economic design and presenting reduced aperture blockage and a uniform field in any plane perpendicular to such line feed.

These and other objects of the invention will become apparent from the following description, taken together with the accompanying drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is an axial section view of an aplanatic spherical reflective system employing a line source feed;

FIG. 2 is a schematic illustration in isometric view of the system of FIG. 1, showing an annular element of illumination upon the spherical reflector, resulting from radiation from an axial element of the line source feed;

FIGS. 3a and 3b are illustrations of exemplary line source feed phase-shifting structures; and

FIG. 4 is a side view of an exemplary feed for use in the system of FIGS. 1 and 2. and showing the approximate ray paths from the feed.

In the figures, like reference characters refer to like parts.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, there is illustrated an axial section view of an aplanatic spherical reflective system employing a spherical reflector 10 and a line source feed 11. Line feed 11 may be a leaky cylindrical waveguide, the longitudinal axis of which intersects the center of curvature O of spherical reflector 10, line feed 11 being fixedly radially of and rotatable about center as a center of rotation. In other words, the line feed 11 lies along a focal axis of reflector 10, the length of the guide being determined by the angular extent of the reflector to be illuminated.

A necessary feature of the device of the invention is means for reducing the spherical aberration associated with the use of a spherical reflector. Such spherical aberration is manifested by the response of spherical surface to a planar wave front, which response is illustrated as the crossing of the focal axis 12 of the reflected rays at points along the focal line FF. By placing the leaky waveguide or line feed Source 11 within the dimension I l and controlling the principal radiation angle 9 therefrom as a function of axial position, such spherical aberration may be corrected. In other words, where a ray 13 of such planar Wave front impinges upon a point 14 of spherical reflector 10, describing an angle t (measured from the center of curvature O of curve 10), the incident angle of such ray relative to the radial line R or normal to surface 10 at point 14 is also the angle t. Ray 13 is then reflected from point 14 to focal point F as ray 15 at a reflected angle which is, of course, equal to the incident angle a. The incident angle 0 of the reflected ray 15 at point F (relative to focal axis 12) is equal to the included angle 2c between ray paths 13 and 15, in view of the fact that alternate interior angles formed by intersection of two parallel lines (12 and 13) by a transversal (15) are equal. In other words, the axial elements comprising leaky guide 11 must demonstrate a principle angle of directivity, 6' as a function of focal line position, corresponding to Ozzy (1) A leaky guide emits radiation at a principal angle 0 in accordance with the expression Where x free space wavelength of the energy mode propagated,

and .t waveguide wavelength of the energy mode propagated.

as is understood in the art, being set forth, for example, as Equation (16l0b) at page 16-7 of Antenna Engineering Handbook by Jasik, published by McGraw-Hill, New

shown in FIG. 4. In other words, the radius of the circular waveguide is made to vary along its length. so as to effect such correction. The correction for such aberration, while necessary, is not sufficient to assure that the feed for the spherical reflector antenna system of FIG. 1 is both focused and mechanically scannable. Other conditions to be fulfilled by the line feed source 11 of FIG. I may be more readily appreciated from a consideration of FIG. 2.

Referring to FIG. 2, there is illustrated a schematic illustration in isometric view of the system of FIG. 1, showing an annular element 16 of illumination upon the spherical reflector 10 resulting from radiation from an axial element of the leaky cylindrical waveguide feed 11 of FIG. 1. Four coordinate systems are depicted and utilized in the explanation of FIG. 2: the origin 0, or center of curvature of spherical reflector 10 and the associated polar coordinate system therefor (R, p), is also the origin of a rectangular coordinate system (X, Y, Z) with the Z axis coincident with the polar focal axis of surface 10 and intersecting surface 10 at point P. A third coordinate system in FIG. 2 also having its origin at point 0, is a cylindrical coordinate system b, Z), p being the radial distance from the Z axis and gb being the azimuth angle measured from the X axis (of the first coordinate system). The fourth coordinate system is a polar coordinate system (r, 0, with origin at focal point F on the line feed (Z axis), r being the radial distance to a point Q on surface 10, being the azimuth angle (in a plane parallel to plane XOY), and 6 being the polar angle (in the plane PF'Q).

As previously indicated in connection with the description of FIG. 1, correction of spherical aberration requires that both of Equations 1 and 2 be satisfied. In other words, the propagation constant 5 21M k in the radiating guide must be chosen so that such equations are satisfied and may be most simply achieved by tapering the radius of the circular cross-section leaky guide or line source feed 11.

Another condition required is that the density of il lumination around an annular ring of illumination 16 (6:241) from a selected radiating axial element of feed 11 (shown in FIG. 2) should preferably be uniform and independent of azimuth angle This condition is met by use of a leaky cylindrical guide with circular symmetry for the axial elements composing line source 11, and making the guide uniformly leaky in the circumferential, or direction. For a cylindrical guide carrying the dominant TE mode, for example, it has been determined that at least six circumferentially evenly-spaced, uniform radiating apertures 22 per circumferential ring are required (as shown in FIGS. 3A and 3B), in order to satisfy this condition. An axial spacing of about onehalf free space wavelength is maintained between adjacent circumferential rings. The circular symmetry of the feed also allows the utilization of any desired state of linear or circular polarization by merely propagating the appropriate polarization in the leaky guide.

With uniform power density around any annular ring 16 (for 0=2\p), a selected variation of power density from ring to ring, over the range of polar angles, =0 to i -p provides control of the antenna aperture amplitude distribution. For example, a tapered amplitude distribution (i.e., high power density near the center of illumination P or \l/ O and decreased power density as ip approaches 1p is preferred to minimize side lobe performance. Thus, those rays from feed line 11 which illuminate the edge l/= of reflector 10 should be weaker than those illuminating the more nearly central portion (it- 0) of reflector 10. As is suggested in FIG. 1 and as may be more clearly seen in FIG. 4, those rays which illuminate the edge of the reflector originate at that terminus of fcedline 11 closest the reflector 10, those rays which illuminate the more nearly central section originating at that terminus furthest from reflector 10.

In general, the attenuation constant for the tapered leaky cylindrical guide or feedline is found to increase with distance along the guide, and may unduly attenuate the radiation level at each axial station. Accordingly, the coupling holes or radiating slots in line source 11 may have to be compensatorily changed in size at successive axial stations (i.e., as a function of Z, in FIG. 2) in order to achieve a desired amplitude distribution.

Although the power density radiated by the feedline 11 at each polar angle, :21p, is preferably uniform in azimuth with or without selected shaping of the antenna aperture amplitude distribution, the total electric field strength will, in the absence of compensation, represent a vector quantity which, in general, does not correspond to maximum antenna aperture efficiency. In other words, the total field is polarized in a complicated way as a function of azimuth angle and is a vector quantity which may be represented by its components, as resolved for example, along the (X, Y, Z) coordinate axes.

It may be shown by the analytical exposition which follows, that the maintenance of a preferred relationship among such components results in a maximum field vector and optimum aperture efiiciency.

The external field radiated by the waveguide feed may be determined by solving the electromagnetic wave equation. The appropriate boundary conditions are that, immediately outside the surface of the feed cylinder the electric field must vary with e and z exactly as do the field components for the wave traveling within the guide. This follows as a result of making the guide uniformly leaky. For any internal TE or TM, mode, and in particular for the dominant TE mode, the internal field components are known to vary sinusoidally with azimuth angle 45. Consequently, the cylindrical components of external field near the guide surface are E,,, E, and E and it is found that E =A cos where A, B and C are complex functions of the coordinates p, z and 0 of the field point, but not of the angle qb. These components are indicated in FIG. 2, where the point F on the feed is depicted as emitting a cone of radiation at the angle 0:211. One particular ray, F'Q A, is shown incident at Q on the reflector 10 and reflected to point A of the aperture. Unlike A and B, C varies inversely as p and becomes vanishingly small at point Q where p is much larger than a the cylinder radius.

In this case the field at Q has only the two spherical components E and E, and these are related to the cylindrical components by From Equation set (3) it follows that E,,=-A cosec 6 cos 4:

E,=B sin (5) This latter equation set expresses one of the polarization requirements, namely that 13 must vary as cos in azimuth, while E, varies as sin,,.

Further conditions imposed are revealed by following this field, after its reflection at Q, to the point A in the aperture. Here it is more convenient to examine the rectangular components of the field. These are related to the spherical components in the following way,

E E,, cos +E, sin

E .,=E, sin E cos 4: (6)

so that, by using Equation set (5),

E =A cosec 0 cos +B sin 45 B (A cosec 0-B) sin cos (7) These latter equations contain the key to the further polarization requirements. Maximum gain and aperture efliciency will occur only when the aperture field is everywhere polarized in one direction, and constant in amplitude as a function of the radial coordinate p. This will be the case only if A and B are chosen so that A cosec 0=B:a constant, K (8) When this is done Equation set (7) shows that E,,=K cos E,,=K sin o The final condition is thus that E, and E radiated by the section of feed at F (FIG. 2) must, apart from their orthogonal azimuthal variations, be equal in magnitude, and opposite in time phase. This requirement, however, is quite opposed to the natural resisting condition existing in a uniformly leaky cylindrical waveguide, which gives rise to E and E, components which are very nearly in mutual time quadrature. This time quadrature relation occurs because the external fields of the leaky guide largely reproduce the behavior of the internal fields, and the components of the latter which give rise to E and E, are inherently degrees out of phase, as is well known in the art.

One aspect of the invention is an external structure, attached to outside of the feed guide, which forces E and E, into the desired (anti-phase) time phase relationship. Equation set (4) shows that the near-field component E gives rise to E, in the far field. The key to the problem is thus to find a means of changing the time phase of either E or E, (relative to the other of them) in the near field, in fact right at the surface of the feed itself. Analysis shows that E must be shifted by 270 degrees, or E, by 90 degrees, and since less phase shift is required for the latter, it is to be preferred.

Two alternative structures effecting the desired time phase condition are indicated in FIGS. 3A and 3B, each of which shows a short section of perforated guide 20 and indicates the directions of the two near-field components E and E, In FIG. 3A metallic electrically conductive discs 21 are attached to the guide cylinder 20 between adjacent rings of slots 22, forming fins extending radially outward, such structural arrangement corresponding to a short axial section of line feed 11 of FIGS. 1 and 2. Adjacent fins 21 thus form a radial transmission line which is excited by a ring of slots 22. Radiation into space now takes place at the outer periphery of the radial transmission lines. Within such radial transmission lines, E propagates as a purely transverse electric and magnetic mode, with a velocity equal to c, the speed of light. However, E, propagates radially outward as a higher mode, with a phase velocity which can be made appreciably greater than 0. By adjusting the radial length of the fins, E, can be made to advance in phase by 90 degrees relative to E This concept has been successfully demonstrated with a length of round copper tubing 3 feet long and 3 inches in diameter at the frequency 2800 mHz. Four rings of holes, spaced 0.53 apart, were cut in the cylinder. Six uniformly spaced holes were used in each ring, the holes initially being round and 1.2 inches in diameter. Precise measurements of the far radiation field of the model showed the expected cos p azimuthal variation in E along with a sin qb variation in E and the two were exactly 90 degrees apart in time phase, but of unequal magnitude. The measurements were repeated after adding fins with a radial extent of 037x as shown in FIG. 3A, with the result that E and B, were observed to be exactly in opposite time phase, but still unequal in magnitude. A slight elongation of the holes in the axial direction to make them oblong then resulted in equalizing the magnitudes of the two components.

In the second structure, shown in FIG. 3B, the fins are replaced by an open network of concentric electrically conductive wire rings or hoops 23a and 23b, supported by radial arms 24 extending outward from the guide surface. This is a cylindrical version of the planar inductive grip. It works on the principle that E being orthogonal to the hoops, propagates through the network without disturbance. 15,, however, is parallel to the conducting Wires and sees an inductive reactance at each ring. The magnitude of this reactance is chosen so as to produce a 45 degree phase shift at each hoop, giving the required 90 degree total phase advance. With appropriate radial spacing between the hoops (approximately 3M8) the reflections from the two can be made to cancel, in order to obtain reflection-less phase shift of E, relative to E The failure to meet all of the above-described require ments has been responsible for the reduced performance, including reduced aperture efliciency and high sidelobe levels, displaced by prior-art spherical reflector antennas. In general, the requirements for spherical aberration correction and for aperture amplitude distribution control have been met in the prior art; however, the requirements for uniform annular power density as a function of and for a preselected field vector component phase relationship for a maximum field vector have not been considered or met, with resulting aperture efficiencies of only 25-50% being common.

In contrast, the subject invention provides means by which all of the above-noted requirements may be closely met, with a resulting aperture efficiency of nearly 100%. The tapering of the leaky cylindrical waveguide line feed effects correction for spherical aberration, the length of the line being selected from considerations of the angular extent of the reflector to be illuminated. The circular symmetry of the cylindrical waveguide feed by means of circumferential rings of at least six radiating apertures each ring (a one-half-wavelength axial spacing between adjacent rings), allows a uniform annular-density field. Preselective shape adjustment or ellipticity of the slots or radiating apertures in each circumferential ring of apertures provides control of the relative magnitudes of the E, and E components to effect a preferred equality be tween them, and the radially extending phase-shift means (externally concentric of the circularly symmetrical line feed and between adjacent circumferential rings of radiating apertures) provides a preferred anti-phase relationship between the magnitudeequalized E, and E, components, whereby a maximum aperture efficiency is achieved.

Accordingly, it is to be appreciated that an improved spherical reflector antenna having a mechanically scannable feed, has been described.

Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of this invention being limited only by the terms of the appended claims.

I claim: 1. A mechanically scannable feed for a spherical reflector type antenna and comprising circularly-symmetrical line feed adapted to be located along a focal line of a spherical reflector for providing an illuminating electric field of uniform annular density, and preselectively disposed as a focused source to correct for spherical aberrations;

electrically conductive radially extending phase-shift means externally concentric with and axially spaced approximately one half wavelength intervals along said line feed means for providing a substantially anti-phase time-phase relationship between the axial and azimuthal components of said electric field;

said line feed means having circumferential rings of at least six uniform radiating apertures per ring, each ring situated intermediate axially adjacent elements of said phase shift means, the aperture areas of successive rings being preselected to provide a tapered antenna aperture amplitude distribution, the uniform apertures of each ring being shaped to equalize the amplitudes of the axial and azimuthal components of said electric field.

2. A mechanically scannable reflector type antenna having a reflector of spherical curvature and leaky line feed means located along a focal line of said reflector and disposed as a focused source to correct for spherical aberration of said reflector,

said line feed comprising a circularly symmetrical feed for providing an illumination field at said reflector of uniform annular density, and

a plurality of microwave phase shift means externally concentric with and axially disposed along said line feed for compensatorily adjusting the normally quadrature time-phase relationship between the axial and azimuthal components of the field to a substantially in-phase relationship.

3. The device of claim 2 in which said phase shift means comprises a preselected one of an electrically con ductive disc-like element and an assembly of mutually concentric electrically conductive discrete wire rings.

4. The device of claim 2 in which said circularly symmetrical feed comprises a circular cross-section guide having circumferential rings of at least six radiation apertures per ring, an axial spacing interval of approximately one-half wavelength occurring between adjacent rings, and in which said phase shift means comprises a preselected one of an electrically conductive disc-like element and an assembly of mutually concentric electrically conductive discrete wire rings.

5. The device of claim 2 in which said line feed is rotatable about a center of curvature of and relative to said reflector.

6. A mechanically scannable, spherical reflector type antenna comprising circularly-symmetrical line feed means located along a focal line of said reflector for providing at said reflector an illuminating electric field of uniform annular density, and disposed as a focused source to correct for spherical aberration; electrically conductive, radially extending phase-shift means externally concentric with and axially spaced approximately one half wavelengtth intervals along said line fed means for providing a substantially anti-phase time-phase relationship between the azimuthal and axial components of said electric field;

said line feed means having circumferential rings of at least six uniform radiating apertures per ring, each ring situated intermediate axially adjacent elements of said phase shift means, the aperture areas of successive rings being preselected to provide a tapered antenna aperture amplitude distribution, the uniform apertures of each ring being shaped to equalize the amplitudes of the azimuthal and axial components of said electric field.

7. Pivotable line source feed means for cooperation with a fixed sperical reflector as a focused source of a scannable pencil beam, comprising a preselectively tapered leaky cylindrical waveguide of a preselected length having axially spaced circumferential rings of radiation apertures presenting a preselectively tapered aperture amplitude distribution, and

a plurality of electrically conductive disc-shaped fins,

externally concentrically mounted upon and extending radially from said waveguide, one such fin being located intermediate adjacent rings of radiation apertures.

8. The device of claim 7 in which the radii of successive ones of said fins are preselected to adjust the time phase between the azimuthal and axial components of energy radiated from said leaky waveguide from a normally mutually quadrature relationship to an anti-phase relationship.

9. Pivotable line source feed means for cooperation with a fixed spherical reflector as a focused source of a scannable pencil beam, comprising a preselectively tapered leaky cylindrical waveguide of preselected length having axially spaced circumferential rings of radiation apertures presenting preselectively tapered aperture amplitude distribution, and

a plurality of electrically conductive wire ring assemblies, externally concentrically mounted upon and extenting radially from said waveguide, one such assembly being located intermediate adjacent rings of radiation apertures.

10. The device of claim 9 in which the radii of successive ones of said assemblies are preselected adjust the time phase between the azimuthal and axial components of energy radiated from said leaky Waveguide from a normally mutually quadrature relationship to an anti-phase relationship.

11. A mechanically scannable reflector type antenna having a reflector of spherical curvature and line feed means located along a focal line of said reflector and disposed as a focused source to correct for spherical aberration of said reflector,

said line feed comprising a circularly symmetrical feed for providing an illumination field at said reflector of uniform annular density, and

a plurality of microwave phase shift means externally concentric with and axially disposed along said line feed for compensatorily adjusting the normally mutual time-phase quadrature relationship between the axial and azimuthal components of the field,

said circularly symmetrical feed comprising a circular cross-section guide having circumferential rings of at least six radiation apertures per ring, an axial spacing of approximately one-half wavelength occurring between adjacent rings.

12. A mechanically scannable reflector type antenna having a reflector of spherical curvature and line feed means located along a focal line of said reflector and disposed as a focused source to correct for spherical aberration of said reflector,

said line feed comprising a circularly symmetrical feed for providing an illumination field at said reflector of uniform annular density, and

a plurality of microwave phase shift means externally concentric with and axially disposed along s aid line feed for compensatorily adjusting the normally mutual time-phase quadrature relationship between the axial and azimuthal components of the field,

said circularly symmetrical feed comprising a circular cross-section guide having circumferential rings of at least six radiation apertures per ring, an axial spacing of approximately one-half wavelength occurring between adjacent rings, each said aperture being slightly elongated in shape to equalize the magnitudes of the azimuthal and axial components of said field.

13. A mechanically scannable reflector type antenna having a reflector of spherical curvature and line source means pivotable about a center of curvature of said reflector and located at a radial distance therefrom corresponding to a focal line, said pivotable line source means comprising a leaky cylindrical waveguide of preselected finite length as a focused source and preselectively tapered to reduce aberration associated with cooperation between said finite source means and said spherical reflector, and

a preselectively tapered cylindrical waveguide having axially spaced circumferential rings of radiating apertures, the aperture areas of successive rings in the direction toward said reflector being successively changed so as to provide preselectively tapered antenna aperture amplitude distribution.

14. A mechanically scannable reflector type antenna having a reflector of spherical curvature and line source means pivotable about a center of curvature of said reflector and located at a radail distance therefrom corresponding to a focal line, said pivotable line source means comprising a leaky cylindrical waveguide of preselected finite length as a focused source and preselectively tapered to reduce aberration associated with cooperation between said finite source means and said spherical reflector, and

polarization sensitive phase shift means axially spaced along said waveguide for adjusting the normally mutually time-phase quadrature relationship between the azimuthal and axial near field polarization components of microwave energy radiated by said waveguide.

15. A mechanically scannable reflector type antenna having a reflector of spherical curvature and line source means pivotable about a center of curvature of said reflector and located at a radial distance therefrom corresponding to a focal line, said pivotable line source means comprising a leaky cylindrical waveguide of preselected finite length as a focused source and preselectively tapered to reduce aberration associated with cooperation between said finite source means and said spherical reflector, said preselectively tapered leaky cylindrical waveguide having axially spaced circumferential rings of radiating apertures; and

a plurality of disc-shaped fins concentrically mounted upon and extending radially from said waveguide, one such fin being located intermediate adjacent ones of said rings.

16. A mechanically scannable reflector type antenna having a reflector of spherical curvature and line source means pivotable about a center of curvature of said reflector and located at a radial distance therefrom corresponding to a focal line, said pivotable line source means comprising a leaky cylindrical waveguide of preselected finite length as a focused source and preselectively tapered to reduce aberration associated with cooperation between said finite source means and said spherical reflector, and

a plurality of mutually-axially spaced line source phase-shift means for maximizing the aperture efficiency of said line source means.

References Cited UNITED STATES PATENTS 2,562,332 7/1951 Riblet 343-771 3,055,004 9/1962 Cutler 343-781 3,224,006 12/1965 Hogg 343781 ELI LIEBERMAN, Primary Examiner U.S. Cl. X.R. 343-771, 912 

